Have you activated the Auto Recharge feature?

To enable the Auto-Recharge funding option, log in to your account and click the 'Billing' tab on the Dashboard. Next click the "[Configure]" link located to the right of the "Auto Recharge status" Section.

In the AUTO RECHARGE SETTINGS section, click on the 'Activate' box. Once this step is done, the Auto Recharge Status will show as 'ON'.